Field of the invention
The invention concerns a method of securing transmission of information via a telecommunication network and in particular an open network such as a network including radio links. The method is more particularly intended to be used to transmit information requiring high levels of reliability and high levels of security at one and the same time by virtue of their nature and/or their function.
There are many applications in which it is essential for the information received to correspond faithfully to that sent. In particular, this is required in many security systems where control information received must correspond to the information that was sent, for example. There is also a routine requirement for the transmission to be totally secure so that the recipients can be assured that information they receive has come without modification from sources authorized to send it. It is often also required for the information to be processed in such a manner that it remains confidential and can be used only by the intended recipients.
A prior art solution enables a receiver unit to determine if information it has received faithfully reproduces the original information transmitted. It involves the use of an error detecting code applied to the digital or digitized data to be transmitted prior to transmission. The data constituting the information is therefore transmitted by the transmitter unit together with the corresponding error code data.
It is then possible for a receiver unit to verify information received using the error code data accompanying the information.
Information is routinely transmitted to a receiver unit in a manner that is systematically redundant to enable it to verify its integrity by comparison when received.
A prior art solution enabling a unit receiving information to determine if the information has come without modification from an authorized transmitter unit involves the transmitter unit employing a first key to encipher information it sends to the receiver unit concerned. The receiver unit then deciphers the information using a second key.
The enciphering of information transmitted in the form of data by a transmitter unit to one or more receiver units can be defined by a formula such as: Ci=T(KjMl) in which Ci corresponds to the encrypted information sent, Kj is the enciphering key, Ml is the information to be enciphered and T is the enciphering function.
The receiver units must decipher the data to recover the information addressed to them. Deciphering is generally obtained by means of an operation that can be defined by the formula: M=T1(Kl, Ci) where M corresponds to the decrypted information, Kl is the deciphering key and T1 is the deciphering function.
As is known in itself, the keys Kj and Kl can be identical to enable information to be enciphered before it is transmitted and deciphered after it is received by symmetrical operations. Choosing different enciphering keys Kj for different transmitter units enables reliable identification thereof by the receiver unit or units to which enciphered information is transmitted.
Assigning a private deciphering key Kl to a single receiver unit is of benefit from the security point of view when one or more transmitter units that communicate with the receiver unit use the same public information enciphering key Kj enabling each transmitter unit to communicate confidentially with the receiver unit. To minimize the risk of security becoming degraded over a period of time, negotiation phases are routinely provided for choosing and changing keys, the negotiations involving the legitimately concerned parties and possibly a supervisor, in particular after a particular phase of use of the keys.
This is useful in particular when information is retransmitted.
Although retransmission has advantages from the reliability point of view, it is not advantageous from the security point of view in that repeating information increases the risk of modification of the information during transmission by an external cause and the risk of violation of the confidentiality of the information by a third party where such confidentiality is required.
Accordingly, the invention consists in a method of securing transmission of information via an open telecommunication network combining the reliability and security imperatives referred to hereinabove.
The method is more particularly intended to set up secure communication between communicating units via a telecommunication network channel.
In accordance with one feature of the invention, data conveying information to be selectively transmitted from a transmitter unit to one or more particular receiver units is encoded prior to transmission using an error detecting code adapted to enable reliable recovery of said information on reception; for security reasons, the error detecting code used for encoding said information is encrypted using an encrypting key defined by application of a time dependent variation law.
In accordance with another feature of the invention and as an alternative to the above process, the data conveying information to be selectively transmitted from a transmitter unit to one or more particular receiver units is encoded prior to sending using an error detecting code adapted to enable reliable recovery of said information on reception and the error detecting code used to transmit information is selected by application of a time dependent variation law.
The invention, its features and its advantages are explained in the following description given with reference to the single figure of the accompanying drawing.